Plate-type heat exchanger with anodic corrosion protection

ABSTRACT

To protect a plate-type heat exchanger ( 1 ) against corrosion due to the attack of sulfuric acid, it is proposed in accordance with the invention that the region through which flows sulfuric acid has at least one metal cathode ( 16, 17 ) and one reference electrode ( 27 ), that at least half the metal plates ( 7 ) have an electric contact ( 23 ) which is connected with the anode ( 21 ) of an electric d.c. voltage source of variable electric voltage, that the metal cathode ( 16, 17 ) likewise is electrically connected with the d.c. voltage source, and that the d.c. voltage source belongs to a potentiostat ( 20 ) which is electrically connected with the reference electrode ( 27 ).

DESCRIPTION

This invention relates to a plate-type heat exchanger comprisingnumerous parallel metal plates, between which there are alternatelyformed permeable cold chambers for cooling liquid and permeable hotchambers for the hot liquid to be cooled, comprising a housingsurrounding the plates, which housing has supply lines and dischargelines for the liquids, comprising a first distribution passage forsupplying the hot liquid to the hot chambers, comprising a seconddistribution passage for supplying the cooling liquid to the coldchambers, and comprising a first collecting passage for discharging thehot, cooled liquid and a second collecting passage for discharging thecooling liquid.

It is the object underlying the invention to protect such plate-typeheat exchanger against corrosion due to the attack of sulfuric acid. Inaccordance with the invention, this is achieved in that the metal platesand the housing are designed for the passage of sulfuric acid as hotliquid and for the passage of water as cooling liquid, that the regionthrough which flows sulfuric acid has at least one metal cathode and onereference electrode, that at least half the metal plates have anelectric contact which is connected with the anode of a d.c. voltagesource of variable electric voltage, that the metal cathode likewise iselectrically connected with the d.c. voltage source, and that the d.c.voltage source belongs to a potentiostat which is electrically connectedwith the reference electrode.

Expediently, a metal cathode is disposed in the first distributionpassage and/or in the first collecting passage, where it gets in directcontact with the sulfuric acid. In particular in the case oflarge-surface metal plates it may be expedient to pass a metal cathodethrough a plurality of hot chambers, which metal cathode is sealedagainst the metal plates and electrically insulated. There is thusobtained contact with the sulfuric acid flowing in the chambers.

The anodically protected metal plates, which are equipped withassociated electric contacts, can for instance also have 2 to 5 electriccontacts per plate, when the plates should rather uniformly be protectedagainst corrosion in all regions. The housing can also have one or moreelectric contacts which are connected with the variable d.c. voltagesource, in order to achieve an anodic protection. Expediently, all metalplates will be protected anodically. By means of the anodic protection,a metal oxide layer is produced on the side against which flows thesulfuric acid, which metal oxide layer prevents the corrosion attack.

The plate-type heat exchanger protected against corrosion in accordancewith the invention can be used for instance in plants for producingsulfuric acid, in which sulfuric acid with a H₂SO₄ content in the rangefrom 90 to 100 wt-% and temperatures in the range from 140° C. to theboiling point must be cooled by indirect heat exchange. The plates maybe made of alloyed steel which apart from iron in particular has thealloying components chromium, nickel and molybdenum.

Embodiments of the plate-type heat exchanger will be explained withreference to the drawing, in which:

FIG. 1 shows a section through the plate-type heat exchanger in aschematic representation,

FIG. 2 shows a variant of the arrangement of a metal cathode, and

FIG. 3 shows a reference electrode in a longitudinal section in aschematic representation.

The plate-type heat exchanger (1) of FIG. 1 has a housing (2) comprisinga supply line (3) and a discharge line (4) for the hot sulfuric acid tobe cooled as well as a supply line (5) and a discharge line (6) for thecooling water. In the housing (2), parallel metal plates (7) areprovided, between which there are disposed hot chambers (8) for thepassage of sulfuric acid to be cooled and cold chambers (8) for thepassage of cooling water. The acid enters through the supply line (3)and first of all gets into a first distribution passage (10), from whereit flows through the hot chambers (8) to a first collecting line (11)and leaves the exchanger (1) through the discharge line (4). The coolingwater enters the exchanger (1) through the supply line (5) and issupplied by the second distribution passage (13) to the cold chambers(9), reaches the second collecting passage (4) and then the dischargeline (6).

A first metal cathode (16) is provided in the first distribution passage(10), and the insulated metal cathode extends through the housing (2).Analogously, a second metal cathode (17) is provided in the firstcollecting passage (11). During the operation of the exchanger (1), bothcathodes (16) and (17) are in contact with the sulfuric acid, whosecorrosion attack must be stopped. In the vicinity of the contact withthe sulfuric acid, the cathodes are made of stainless steel, forinstance, which has a high resistance to hydrogen embrittlement, andoutside the housing (2) they are connected with the negative poles (19)and (19 a) of a potentiostat (20) by electric lines (18) or (18 a). In amanner known per se, the potentiostat has a variable d.c. voltagesource, whose positive pole (21) is connected with the electric contacts(23) of the metal plates (7) to be protected via the electric line (22).The housing (2) also has an electric contact (23 a), in order to obtainan anodic corrosion protection. Each of the plates (7) to be protectedcan have a plurality of electric contacts (23) connected with thepositive pole (21), e.g. 2 to 5 electric contacts per plate. Mostly, itis expedient to dispose the contacts (23) at the edges of the plates, inorder to achieve a constructionally simple realization of the anodicprotection.

The potentiostat (20) has a connection (25) for an electric line (26)which leads to a reference electrode (27). This reference electrode (27)provides the measurement basis for the potentiostat (20), and in amanner known per se it may constitute e.g. a calomel electrode, aHg/Hg₂SO₄ electrode, or a cadmium bar as shown in FIG. 3. Surrounded byan electric insulation (31), the cadmium bar (30) is provided in ahousing (32) which has a diaphragm (33). This diaphragm is permeable forthe sulfuric acid, so that the cadmium bar (30) is constantly immersedin the sulfuric acid flowing past the same. The potential developedthereby is supplied to the potentiostat (20) by the electric line (26).

In the case of large metal plates the anodic protection in the middleregion of the plate surface can possibly be smaller than in the vicinityof the plate edge. To provide sufficient anode current for the desiredcorrosion protection in this case as well, it may be expedient to pass ametal cathode through the middle region of the plates (7), as isschematically represented by means of FIG. 2. The cathode constitutes ametal bar (16 a), and the sealed and electrically insulated cathodeextends through the plates (7). In this way, the anode current necessaryin the plate region susceptible to corrosion can precisely bedetermined.

1. A plate-type heat exchanger comprising numerous parallel metalplates, between which there are alternatively formed permeable coldchambers for cooling liquid and permeable hot chambers for the hotliquid to be cooled, a housing surrounding the plates, which housing hassupply lines and discharge lines for the liquids, a first distributionpassage for supplying the hot liquid to the hot chambers, a seconddistribution passage for supplying the cooling liquid to the coldchambers, and a first collecting passage for discharging the hot, cooledliquid and a second collecting passage for discharging the coolingliquid, the metal plates and the housing being designed for the passageof sulfuric acid as hot liquid and for the passage of water as coolingliquid, the region through which flows sulfuric acid having at least onemetal cathode and one reference electrode, at least half the metalplates having a plurality of electric contacts connected with the anodeof a d.c. voltage source of variable electric voltage, the metal cathodelikewise being electrically connected with the d.c. voltage source, andthe d.c. voltage source belonging to a potentiostat which iselectrically connected with the reference electrode.
 2. The plate-typeheat exchanger as claimed in claim 1, wherein a metal cathode isdisposed in the first distribution passage.
 3. The plate-type heatexchanger as claimed in claim 1, wherein a metal cathode is disposed inthe first collecting passage.
 4. The plate-type heat exchanger asclaimed in claim 1, wherein a metal cathode extends through a pluralityof hot chambers, which metal cathode is sealed against the metal platesand electrically insulated.
 5. The plate-type heat exchanger as claimedin claim 1, wherein a plurality of metal plates have 2 to 5 electriccontacts per plate.